The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through protection devices to designated branch circuits supplying one or more loads. These overcurrent devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed.
Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows their reuse. Typically, circuit breakers interrupt an electric circuit due to a disconnect or trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current. A ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor which could be caused by a leakage current or an arcing fault to ground.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor or at a faulty contact or connector (“series” arcs), or, between two conductors supplying a load, or between a conductor and ground (“parallel” arcs). However, arcing faults may not cause a conventional circuit breaker to trip. Arcing fault current levels may be reduced by branch or load impedance to a level below the trip curve settings of the circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or person will not trip a ground fault protector.
There are many conditions that may cause an arcing fault. For example, corroded, worn or aged wiring, connectors, contacts or insulation, loose connections, wiring damaged by nails or staples through the insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and/or cause the conductor to reach an unacceptable temperature.
The present invention concerns detection of low current series arcs that occur within the normal operating range of household appliances and/or low current series arc fault detection that is within the handle rating of the protection device (e.g., a household circuit breaker).
U.S. Pat. No. 6,008,973 (commonly assigned) based largely on slope changes in the load current and does not look at specific sub harmonic, broadband noise content and/or utilize load recognition. In contrast, this invention uses sub harmonic content, broadband noise and load recognition to set thresholds and select arc signatures.
More specifically, load recognition and periodic or repetitive sub-harmonic and high frequency content are used in the determination of “series arc” or “no arc” conditions in the protected circuit/load.
The invention may be applied to residential, commercial, industrial applications for circuit protection against series arc faults.